Epoxy resin compositions are useful to encapsulate electronic components, and as structural adhesives, and the like. Reinforced epoxy resin composites having high strength to weight ratios have found extensive use in the aircraft and aerospace industries, and in other applications where strength, corrosion resistance and light weight are desirable. For instance, fiber resin matrix materials have replaced aluminum and other metals in primary and secondary structures of modern military and commercial aircraft. Sporting equipment such as tennis rackets and golf clubs have also adopted fiber resin materials successfully.
Epoxy resin compositions and fiber modifications are abundant. Since the advent of fiber resin matrix materials, much effort has been expended in improving their properties and characteristics, including the development of many different curing systems.
Amine and polyamine curing agents have received wide acceptance, but the toxicity, low solubility, high exotherm and variable curing rates seen with the most commonly used amines, such as m-phenylenediamine, 4,4'-diaminodiphenyl methane and 4,4'-diaminodiphenyl sulfone, has made further improvement desirable. In particular, for aircraft structural applications, epoxy resins cured with available curing agents are either too brittle or do not have sufficient strength and stiffness under hot/wet conditions. It is disclosed in U.K. Pat. No. 1,182,377, which is incorporated herein by reference, that certain aromatic polyamines are effective as curing agents for a variety of polyepoxides, and the resulting cured compositions are useful as films, moldings, coatings and glass-reinforced laminates. There is no indication in the properties presented in the U.K. Patent that the curing agents exemplified therein will produce the combination of toughness and strength under hot/wet conditions essential for use in the abovementioned structural applications.
In U.S. Pat. No. 3,932,360, diamine cured polyurethane products are described, in which the diamines are of the formula, e.g., ##STR1## wherein n is an integer from 2 to 12. This '360 patent does not deal with curing compounds having more than one epoxide group per molecule.
In Gillhan et al, Organic Coatings and Applied Polymer Science Proceedings, Vol. 46, p. 592-598, March-April, 1982, polyepoxides cured with diamines of the immediately preceding formula (n is 3), are described.
The present development relates to curable epoxy resin compositions. In one of its aspects, it provides fiber resin matrixes comprising reinforcing filaments in a heat-curable epoxy resin composition comprising an epoxy prepolymer and a novel family of aromatic polyamine curing agents. No member of this novel family of curing agents is specifically exemplified in the U.K. Patent. The invention provides neat resin formulations having, after cure, improved physical properties, e.g., higher elongation and satisfactory hot/wet modulus. The epoxy compositions of the present invention, cured with filaments, exhibit improved interlaminar toughness and residual compression strength after impact, while maintaining compression strength under hot/wet conditions.